@Sergey Sharybin (sergey) just switch cube to Edit mode, it’s supposed to show weights with this option set, works in 2.74 but not in current master… Probably some regression in drawing code (or maybe derivedmesh evaluation).

Can you describe in more details which exact configuration is different between 2.74 and 2.75a?

Both versions are reset to defaults if that's what you mean.
In the first image (2.74 in solid mode) the weight visualization is ok, however in the second image (2.75a in solid mode) the weight visualization is not working at all.
In the third image (2.75a matcap) there is a slightly color overlay showing the weights, i think that is correct because 2.74 with matcap presents the same behavior.
The problem only happens in 2.75 when the viewport is set to solid mode.

@Antony Riakiotakis (psy-fi) does this ring some bells for you? reading code, it looks like weight paint (aka CD_PREVIEW_MLOOPCOL) drawing takes two different paths, depending on whether we have an EditMesh DM or not - but both cases are broken currently (nothing happens with an EditMesh DM, and with regular CDDM & co, faces are drawn all yellow-ish, selection color)…